Firearm firing mechanisms generally include a number of components that cooperate to hold a spring-loaded hammer in a cocked position and then selectively release the hammer. The firearm is fired or discharged when the force of the released hammer is applied, directly or through an intermediate device, to an ammunition cartridge loaded in the firearm. The components for holding the hammer in a cocked position and then releasing the hammer as desired may be referred to as a trigger group. In addition to the hammer itself, a trigger group generally includes a trigger component having a finger lever or trigger that a user pulls to release the hammer, and further includes a sear which may be a separate component or integrally formed with the trigger component. Some trigger groups also include other components such as a disconnector for example. The disconnector is used in semi-automatic firearms to catch the hammer as it rebounds after firing and to hold the hammer in a cocked position until the shooter can release the trigger and thereby reset the trigger group to the “ready to fire position.” The disconnector is required for this function because semi-automatic firearms usually cycle so quickly that it is physically impossible for the shooter to release the trigger quickly enough after a discharge to allow the sear to recapture the hammer in the cocked position.
Both the hammer component and trigger component are commonly mounted for rotation in the firearm. The hammer is mounted on a hammer pin to facilitate the desired rotation, while the trigger component is mounted on a trigger pin. Each pin is retained in a respective pin receptacle formed in the firearm. Each such pin receptacle is defined by a first opening on one side of the firearm and a second opening on the opposite side of the firearm. A hammer spring is included in the trigger group to, among other things, bias the hammer forward to a striking position. The trigger component is also spring biased to provide resistance against pulling the trigger and to return the trigger after it has been pulled.
It may be desirable to modify or replace trigger group components in a firearm. This is particularly true for firearms used in competition. Such competition firearms may be fitted with special trigger group components designed to improve firearm performance or operational characteristics, or to suit the preferences of the particular user. Also, different competitions or firearm applications may require different trigger group characteristics. However, due to the relatively small components in the trigger group, the spring loading of components, the close tolerances between components, and the small area in the firearm allotted for trigger group components, a trigger group may commonly be installed only by a skilled gunsmith using specialized tools in order to ensure safe, proper, and reliable trigger group functioning. Due to the difficulty in changing out trigger group components, it is common for many competition shooters to have several different complete firearms each with a different trigger group setup for a particular competition or portion of a competition. Maintaining several complete firearms greatly increases the cost of competitive shooting.
Some prior art original equipment manufacturer (OEM) firearm designs include a detachable housing that houses trigger group components. The housing may be attached to the firearm frame with screws or with pins that extend transversely through the housing and are supported at either end by receptacles or bearing openings in the firearm frame. These prior art OEM trigger group housings attach from the bottom of the firearm and include a trigger guard and surfaces that actually form part of the exterior of the fully assembled firearm. Prior art OEM trigger group housings also house safety components that cooperate with the trigger group components. These structures and surfaces associated with the prior art OEM trigger group housings limit their use to firearms specifically designed for such housings.